Regulation or control of a fusion process in a three-phase current arc furnace

ABSTRACT

A method for regulation or control of the melting process in a three-phase arc furnace having at least three electrodes whose height is adjustable individually and independently of one another. Each phase of the three-phase current which feeds the three-phase arc furnace is assigned at least one electrode. The temperature of the three-phase arc furnace is monitored, particularly at points which are at risk of overheating, such as the walls of the three-phase arc furnace in the vicinity of the electrodes. When the three-phase arc furnace reaches a critical temperature in the vicinity of an electrode, the power emission from this electrode is reduced in such a manner that overheating of the three-phase arc furnace is prevented, and in that the power emission from the other electrodes or from some of the other electrodes is increased in such a manner that the total power emission from the electrodes is a maximum at a predetermined voltage.

FIELD OF THE INVENTION

The present invention relates to a method and to a device for regulationand control of the melting process in a three-phase arc furnace having aleast three electrodes whose heights are adjustable individually andindependently of one another.

BACKGROUND INFORMATION

In three-phase arc furnaces, scrap metal is melted by means ofelectrical energy, with the conversion of the electrical energy intothermal energy that is required for the melting process taking place inthe three arcs which burn between the electrode tips and the material tobe melted. For process control, the operating point of the arc furnacecan be varied by step-by-step adjustment at the voltage supplied to thearc furnace via a furnace transformer, as well as by step-by-step,separate adjustment, of the distances between the electrode tips and thematerial to be melted. The chosen operating point is maintained bycontrolling the distances between the electrodes and the material to bemelted. This is generally done by means of impedance regulation bforming an actual impedance value for each electrode from continuouslymeasured electrical variables such as the phase voltage and the phasecurrent, and using the error between this actual impedance value and apredetermined set impedance value to define a manipulated variable foradjusting the height of the respective electrode.

In order to match the operating point of the arc furnace to the variablerequirements for furnace operation during the melting process, thoseoperating points which are respectively assigned to a voltage level onthe furnace transformer from predetermined characteristics for furnaceoperation are stored, in the case of a method that is described inEuropean Patent No. 0 036 122, as set impedance values in a digitalmemory. The furnace operator then manually selects suitable operatingpoints from this digital memory during the melting process.

The presetting of the operating points and set impedance values forsetting the height of the electrodes has until now been carried out as afunction of previously defined process phases (first bucket, secondbucket, refining) and sections within these process phases (placing theelectrodes against the cold scrap, melting down, final melting on a flatmelt bath), with the set impedance values in each case beingpredetermined and fixed during these sections. This previous procedureis based on the assumption that the conditions within a section remainsufficiently constant and are reproducible from charge to charge.Steady-state balancing of the three-phase arc furnace is also carriedout on the basis of similar principles.

On the basis of relatively detailed knowledge about the behavior ofalternating-current arcs in three-phase arc furnaces, one may, however,assume that the preconditions for steady-state setting of the arcfurnace do not occur. In fact, it should be assumed that the conditionsin the individual arcs vary even during the sections of the processphases and, in particular, unbalanced operating conditions can occurwith respect to the arrangement of the electrodes in the three-phase arcfurnace. If the set impedance values are then set in a balanced andsteady-state manner on the assumption of balanced conditions in the arcfurnace, then, if the furnace conditions are actually asymmetric, thisleads to different manipulated variables which then produce arcs ofdifferent length via the adjustment of the heights of the electrodes.Optimum utilization of the power supply to the arc furnace is notpossible in this case.

SUMMARY

AN object of the present invention is to increase the power emissionfrom the electrodes.

This object is achieved according to the present invention by providinga method and a device in which when the three-phase arc furnace reachesa critical temperature in the vicinity of an electrode, the poweremission from this electrode is reduced in such a manner thatoverheating of the three-phase arc furnace is prevented and the poweremission from the other electrodes or from some of the electrodes isincreased in such a manner that the total power emission from theelectrodes is a maximum at a predetermined voltage. In this case, theimpedances in the current paths of the electrodes are advantageouslyoptimized in the sense of maximizing the total power emission from theelectrodes, wherein, when the three-phase arc furnace reaches a criticaltemperature in the vicinity of an electrode, the power emission fromthis electrode is limited to a maximum value, and wherein this maximumvalue is taken into account in the sense of a secondary condition in theoptimization, as described in, for example, in German Patent No. 44 15727. If the supply voltage to the electrodes is not reduced in thiscase, provided a second critical temperature limit is not exceeded, anincrease in the production upwards of about 7 to 12% is achieved.

This optimization process is carried out on-line in a particularlyadvantageous manner.

In an advantageous refinement of the present invention, if thetemperature of the three-phase arc furnace in the vicinity of anelectrode is above a first critical temperature limit and below a secondcritical temperature limit, a redistribution of the power emission fromthe individual electrodes is carried out, with the supply voltage to theelectrodes being kept constant. If the temperature of the three-phasearc furnace in the vicinity of an electrode is above the second criticaltemperature limit, a redistribution of the power emission from theindividual electrodes is advantageously carried out, with the supplyvoltage to the electrodes being reduced.

In another advantageous refinement of the present invention, if thetemperature of the three-phase arc furnace in the vicinity of anelectrode is above a third critical temperature limit, which is abovethe second critical temperature limit, the supply voltage to theelectrodes is greatly reduced. This version is shown in FIG. 5.

The method according to the invention operates particularly reliably ifpreviously estimated temperature values are used instead of the measuredor estimated values for the temperature of the three-phase arc furnace.This previous estimation is advantageously carried out using a gradientmethod or a temperature model, in which case the temperature model maybe an analytical model or a neural network.

In another advantageous refinement of the present invention, thetemperature of the three-phase arc furnace is regulated in such a mannerthat the real power emitted from an electrode is reduced when thetemperature of the three-phase arc furnace exceeds a set temperature,and the real power emitted is increased when the temperature of thethree-phase arc furnace is less than a set temperature. In this case,the set temperature advantageously corresponds to a critical temperaturelimit, which is in the region of a three-phase arc furnace temperaturethat is just still permissible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of a three-phase arc furnacehaving an upstream furnace transformer and a regulation arrangementaccording to the present invention.

FIG. 2 shows a simplified electrical equivalent circuit of thethree-phase arc furnace according to the present invention.

FIG. 3 shows a power regulation system according to the presentinvention.

FIG. 4 shows an alternative embodiment of a power regulation system. and

FIG. 5 shows an impedance regulation system according to the presentinvention.

DETAILED DESCRIPTION

FIG. 1 shows a three-phase arc furnace in which three graphiteelectrodes 1, 2 and 3 project through openings in a furnace cover 4 intoa furnace vessel 5, into which scrap metal 6 is introduced as thematerial to be melted. The electrodes 1, 2 and 3 are connected to athree-phase power supply 8 via a furnace transformer 7 which can beadjusted step-by-step. Arcs 9 are formed between the tips of theelectrodes 1, 2 and 3 and the material 6 to be melted, in which arcs 9the electrical energy supplied to the arc furnace is converted intothermal energy, in order to melt the material 6 to be melted.

Each electrode 1, 2 and 3 is equipped with a regulation arrangement.Since the regulation arrangements are all designed in exactly the sameway, only the regulation arrangement for the electrode 1 will bedescribed in more detail. In the same way as the other respectiveelectrodes 2 and 3, the electrode 1 is mounted on a supporting arm 17whose height is adjustable via a hydraulic positioning apparatus 10having an electrohydraulic actuator 11, by means of which the arc lengthcan be set. The actuator 11 is connected to the output of an impedanceregulator 12.

The phase current i₁ flowing through the electrode 1 and the phasevoltage u_(1M) between the associated secondary terminal of the furnacetransformer 7 and the star point formed by the furnace vessel 5 with thematerial 6 to be melted are detected, respectively, via a currenttransformer 13 and a voltage transformer 14 and are converted, in ameasurement device 15, into an actual impedance value Z₁ for therelevant furnace phase. This actual impedance value Z₁ is compared at anaddition point 16 with a predetermined set impedance Z₁ * for therelevant phase. The control error ΔZ₁ obtained in this way is suppliedas an input variable to the impedance regulator 12 which produces anactuating signal S₁, in order to adjust the height of the electrode 1,as a function of the control error ΔZ₁.

The set impedance values Z₁ *, Z₂ * and Z₃ * are determined by means ofan impedance optimizer 30 according to the invention. The impedanceoptimizer 30 calculates the set impedance values Z₁ *, Z₂ * and Z₃ * insuch a manner that the power emission from the electrode 1, 2, 3 is amaximum. The temperature T of the three-phase arc furnace is monitoredby means of a temperature measurement device 31. If the temperature T ofthe three-phase arc furnace exceeds a first critical temperature limitin the vicinity of an electrode 1, then the real power emitted from theelectrode 1 is limited by the real-power emission from at least one ofthe other electrodes being raised, while the secondary voltage of thetransformer 7 is not changed. The set impedance values Z₁ *, Z₂ * andZ₃ * are in this case calculated in such a manner that the total poweremission is a maximum without the maximum permissible real-poweremission from electrode 1 being exceeded. If the first criticaltemperature limit is exceeded in the vicinity of all the electrodes 1, 2and 3, then the secondary voltage of the transformer 7 is reduced. Thesecond voltage of the transformer 7 is also reduced if a second criticaltemperature limit is exceeded in the vicinity of an electrode 1.

The impedance optimizer 30 and the impedance regulator areadvantageously implemented on a control and regulation device 32.

As the simplified electrical equivalent circuit of a three-phase arcfurnace in FIG. 2 shows, the three-phase arc furnace represents aresistive-inductive load which is connected in star, is unbalanced,varies with time, and whose free star point M is formed by the material6 to be melted. The electrical variables associated with the individualphases are provided with the indices k=1, 2, 3 corresponding to theassociated electrodes in FIG. 1. The concatenated voltages between thefurnace phases are denoted by u₁₂, u₂₃ and u₁₃, and the individual phasecurrents are denoted i₁, i₂ and i₃. The cable resistances R_(L1), R_(L2)and R_(L3) and the cable reactances X_(L1), X_(L2) and X_(L3) formed bythe electrodes 1, 2 and 3 and their supply cables can be determined bymeasurement in a short-circuit test, that is to say by placing theelectrodes 1, 2 and 3 against the scrap metal 6.

The electrical behavior of an arc 9 is described by a non-linear anddiscontinuous current/voltage characteristic whose profile is dependenton the distance between the electrode tip and the melt. In theillustrated equivalent circuit, each arc 9 is replaced respectively byan arc resistance R_(b1), R_(b2), Rb₃ and an arc reactance X_(b1),X_(b2), X_(b3). The arc resistance and reactance of each individual arc9 are dependent not only on the length of the associated arc 9, but alsoon the arc lengths of the respective other arcs. Thus, if, for example,the distance between the electrode 1 and the melt 6 is varied, then thearc resistances R_(b2) and R_(b3) and arc reactances X_(b2) and X_(b3)also vary as well as the arc resistance R_(b1) and the arc reactanceX_(b1).

The real power which is consumed in the individual arcs can becalculated with the aid of the complex alternating-current calculationfor the linearized equivalent circuit shown in FIG. 2. If the reactancesX_(Lk) and X_(bk) in each phase are in this case combined to form anoperating reactance X_(k) and the resistances R_(Lk) and R_(bk) arecombined, ignoring the cable loss resistances R_(Lk), to form anoperating resistance R_(k) where k=1, 2, 3, then the real power levelsP_(k) in the individual arcs 9 are as follows:

P₁ =P₁ (U₀, R₁, R₂, R₃, X₁, X₂, X₃)

P₂ =P₂ (U₀, R₁, R₂, R₃, X₁, X₂, X₃)

P₃ =P₃ (U₀, R₁, R₂, R₃, X₁, X₂, X₃)

In this case U₀ denotes the root-mean-square value of the concatenatedvoltage set via the furnace transformer 7 for the three-phase system,which is assumed to be balanced. The total real power consumed in thearc 9 is then given by

P=ΣP_(k) (U₀, R₁, R₂, R₃, X₁, X₂, X₃) where k=1, 2, 3.

As already mentioned, the resistances R_(k) and reactances X_(k) in eachfurnace phase vary with the arcs length, although the relationshipbetween the resistances R_(k) and the reactances X_(k) is not linear. Ifthis relationship is known, the number of unknown variables on which thetotal real power in the arcs depends is reduced to four. In order tomaximize the total real powe P, these four variables can be regulatedwith the aid of the four available manipulated variables, namely thetransformer voltage U₀ and the three manipulated variables S₁ S₂ and S₃for adjusting the height of the individual electrodes 1, 2 and 3. Inother words, if the non-linear relationships between the resistancesR_(k) and reactances X_(k) can be described by a non-linear functionX_(k) =NN_(k) (R₁, R₂, R₃), an optimum resistance triplet R_(1opt),R_(2opt), R_(3opt) occurs at the maximum of the total real power P, fromwhich, in turn, the optimum set impedances Z₁ *, Z₂ * and Z₃ * forimpedance regulation, and thus the height adjustment of the electrodes1, 2, 3, can be calculated using the functions NN_(k).

FIG. 3 shows one example of the regulation for the melting process inthe three-phase arc furnace, based on a block diagram. The arc furnace18 is fed from the furnace transformer 7 with the concatenated voltagesu₁₂, u₂₃ and u₁₃. The distances between the electrodes 1, 2 and 3 andthe material 6 to be melted are set as a function of the control signalsS₁, S₂, S₃ which are provided by impedance regulators 12 which areassigned individually to the electrodes 1, 2 and 3 and are in this casecombined in an impedance regulation arrangement 19. The impedanceregulation arrangement 19 in this case produces the control signals S₁,S₂, S₃ as a function of the control errors between predetermined setimpedance values Z₁ *, Z₂ *, Z₃ * and measured actual impedance valuesZ₁, Z₂, Z₃ for the arc furnace 18. The phase voltages u_(1M), u_(2M),u_(3M) and phase currents i₁, i₂, i₃ are sampled continuously with ahigh sampling rate in a measurement device 20, and are pre-processed intime with the basic oscillation period. In this case, the measuredvariables u_(kM) and i_(kM), where k=1, 2, 3, are used to calculate theroot-mean-square values of the phase voltages U₁, U₂, U₃, theroot-mean-square values of the phase currents I₁, I₂, I₃, the phasereal-power levels P₁, P₂, P₃ and the furnace impedances, that is to saythe actual impedance values Z₁, Z₂, Z₃ where Z_(k) =U_(k) /I_(k). Theassociated actual resistance values R₁, R₂, R₃ where R_(k) =P_(k) /I_(k)² and the actual reactance values X₁, X₂, X₃ where X_(k) =(Z_(k) ²-R_(k) ²)^(1/2) are also calculated together with the actual impedancevalues. Finally, the thermal wall loads V₁, V₂₁ V₃, caused by the arcs9, in the arc furnace 18 are also determined using, for example,

V_(k) =P_(k) ² /I_(k) ²

The continuously predetermined actual resistance values R₁, R₂, R₃ aresupplied to a neural network 21 which is used to determine, by the useof a learning process, the relationships between the measured actualresistance values R₁, R₂, R₃ and the actual reactance values X₁, X₂, X₃.To do this, the neural network 21 produces three network responsesX_(NN1), X_(NN2), X_(NN3) on the output side, which are compared in acomparison device 22 with the associated actual reactance values X₁, X₂,X₃. Depending on the comparison results, the network parameters α_(NN1),α_(NN2), α_(NN3) of the neural network 21 are varied in a learningalgorithm 23, in the sense of reducing the errors between the networkresponses X_(NNk) and the actual reactance values X_(k). Therelationships between the resistances R_(k) and reactances X_(k) arelearnt in this way, and the knowledge thus obtained is updatedcontinuously, as a function of the ongoing process activity. Instead ofa single neural network, three different neural networks may also beprovided, to the input sides of each of which all the actual resistancevalues R₁, R₂, R₃ are supplied, and from which each neural networksupplies in each case one network response X_(NNk).

The learnt relationship X_(k) =NN_(k) (R₁, R₂, R₃) is transmitted to acomputation device 24 in which the total real power P of the arcs 9 isrepresented, on the basis of the equivalent circuit shown in FIG. 2, asa function of the root-mean-square value of the concatenated voltage U₀,of the resistances R₁, R₂, R₃ and of the reactances X₁, X₂, X₃. For apredetermined voltage level U₀, the maximum real power P_(max) isobtained on the basis of the condition ∂P/∂R₁ =0, ∂P/R₂ =0 and ∂P/∂R₃=0. The solution to these equations gives the resistance valuesR_(1opt), R_(2opt), R_(3opt) where R_(kopt) =R_(kopt) (X₁, X₂, X₃) atthe maximum real power P_(max). In a next step, the relationship X_(k)=NN_(k) (R₁, R₂, R₃) is used as the basis for obtaining the reactancesX_(1opt), X_(2opt), X_(3opt) associated with the resistances R1opt,R2opt, R3opt and thus, finally, the set impedance values Z₁ *, Z₂ *,Z₃ * where Z_(k) *=(R_(kopt) +X_(kopt) ²)^(1/2).

The condition stated above, that the derivative of the total real powerP tends to zero at the optimum on the basis of the resistances R_(k), ispermissible only if there are no boundary conditions and the globaloptimum has been reached. However, since, as a rule, the phase currentsi_(k), the thermal wall load V_(k) and the resistances Rk are subject toboundary conditions, the optimization task consists of maximizing thetotal real power P while complying with the boundary conditions. Theboundary conditions may be any non-linear functions of the variables U₀,R₁, R₂, R₃, X₁, X₂, X₃, although the relationships between the variablesR_(k) and X_(k) are known from the knowledge contained in the neuralnetwork 21. In the block diagram shown in FIG. 3, the determined thermalwall loads V₁, V₂, V₃ are used in computation unit 25 to determine thesecondary conditions for maximizing the total real power P, and thesesecondary conditions are transmitted to the computation device 24.

FIG. 4 shows an alternative embodiment. In this case, temperaturemeasured values T, which are determined using a measurement device 26,are used instead of the thermal wall loads. If the temperature measuredvalues or the determined wall loads exceed specific critical values,then, in a particularly advantageous refinement, the voltage step U₀ towhich the fernace transformer 7 is set is not reduced. The voltage stepU₀ is not reduced until a second critical temperature limit has beenexceeded, which is above the first critical temperature limit.

In another alternative embodiment, not only are the thermal wall loadsV₁, V₂, V₃ determined, but the temperature measured values T aredetermined, using a measurement device 26.

FIG. 5 shows an impedance regulation system according to the presentinvention. As a rule, three-phase arc furnaces have cooling boxes on theupper part of the wall, whose task is to dissipate that portion of theradiation energy which is radiated to the wall rather than into thesteel bath. These cooling boxes typically operate with evaporationcooling or with water cooling. If the radiation energy which is radiateddirectly to the cooling boxes becomes excessive or is too great over acertain time period, then the cooling boxes can no longer completelydissipate the energy. In the case of evaporation cooling, this isindicated by a reduction in the flow rate and, in the case of watercooling, by a rise in the water return temperature.

Overloading of the cooling boxes shall be avoided since, on the onehand, the cooling boxes are damaged and, on the other hand, overloadingmay also be regarded as a measure of wear of the refractory material onthe lower part of the wall of the arc furnace. For this reason, anyregulation system must react to heat acting on the cooling boxes. Untilnow, it has generally been done by reducing the secondary voltage, andthus also by noticeably reducing the total real power.

It has been found that, normally, heat does not act simultaneously onall the cooling boxes, but generally only on one cooling box or oncooling boxes which are positioned in the area of one of the threeelectrodes. (Each cooling box is associated with that electrode which itis closest to). The regulation system according to the present inventionreacts to heat acting only in the area of an electrode by reducing theradiation power of the relevant electrode while keeping the secondaryvoltage constant, and by emitting at least a portion of this power tothe two other electrodes. This is achieved by a deliberate change to thethree phase impedances, that is to say the impedances in the currentpaths to the electrodes. The three phase impedances are in this case setusing an optimization method. In this case, the total real power ismaximized using the boundary or secondary condition that the power ofthe relevant electrode is reduced by a specific reduction factor F withrespect to the power and time at which the heat action took place.

As is shown in FIG. 5, the regulation system in this case distinguishesbetween four different cases:

1. If the heat action is less than a constant DQ₋₋ K1 which correspondsto a first critical temperature limit, then no action is taken and theregulation system sets the phase impedances such that the total realpower of all three electrodes is maximized.

2. If the heat action is between the constants DQ₋₋ K1 and DQ₋₋ K2,which corresponds to a second critical temperature limit, then the poweris redistributed from the relevant electrode to the two otherelectrodes, without the secondary voltage being reduced. The power tothe relevant electrode is reduced by a reduction factor which isdependent on the intensity of the heat action. The reduction factor F isadvantageously calculated using, for example, ##EQU1## where W denotesthe action of the heat.

3. If the heat action is between DQ₋₋ K2 and DQ₋₋ K3, which correspondso a third critical temperature limit, then, owing to the very severeaction of the heat, not only is the secondary voltage reduced, but thepower to the relevant electrode is also redistributed to the otherelectrodes. For example, the reduction factor could be F=0.9.

4. If the heat action is greater than DQ₋₋ K3, then the secondaryvoltage is reduced, as a type of emergency shutdown, to a very lowtransformer step.

The value of the reduction factor F and the constants DQ₋₋ K1, DQ₋₋ K2and DQ₋₋ K3 are matched to the nature of the cooling boxes and to theconditions in the arc furnace. Thus, for example, for evaporationcooling, the constants may be chosen such that they occur at a flow ratereduction of 10, 20 and 50%, as is shown in FIG. 1. For water coolingthey could, for example, advantageously be chosen to occur at returntemperatures of 80, 90 and 105° C.

The reduction in the power to the relevant electrode by the factor F iscontinued until the action of the heat has been reduced.

The critical temperature limits may therefore be temperatures in theactual sense, such as coolant temperatures or equivalent variables, suchas the reduction in the flow rate in the case of evaporation cooling.

The advantages of the present invention are, on the one hand, theincrease in the production rate and the reduction in the switching stepson the supply transformer. In addition to the considerable costreduction from an increase in the production rate, the reduction in theswitching processes in the supply transformer also has a major economicadvantage. The switching processes in the supply transformer are themajor influencing variables that reduce the life of the transformer andincrease the maintenance frequency. The present invention thus reducesthe frequency of maintenance work on the supply transformer. This inturn leads to considerable cost savings, and improves the availabilityof the three-phase arc furnace.

LIST OF REFERENCE SYMBOLS

1,2,3 Electrodes

4 Furnace cover

5 Furnace vessel

6 Scrap metal

7 Furnace transformer

8 Three-phase power supply

9 Arc

10 Positioning apparatus

11 Electrohydraulic actuator

12 Impedance regulator

13 Current transformer

14 Voltage transformer

15 Measurement device

16 Addition point

17 Supporting arm

18 Arc furnace

20 Measurement device

21 Neural network

22 Comparison device

23 Learning algorithm

24 Computation device

25 Computation unit

26 Measurement device

30 Impedance optimizer

32 Control and regulation device

What is claimed is:
 1. A method for regulation or control of a meltingprocess in a three-phase arc furnace having at least three electrodes,each of the electrodes having a height which is adjustable individuallyand independently of one another, comprising the steps of:assigningphases of a three-phase current which feeds the three-phase arc furnaceto at least one of the electrodes; monitoring a temperature of thethree-phase arc furnace; and when the three-phase arc furnace reaches acritical temperature in the vicinity of one of the electrodes, i)reducing power emission from the one of the electrodes to preventoverheating of the three-phase arc furnace, and ii) increasing poweremission from at least another of the electrodes so that total poweremission from the electrodes is a maximum at a predetermined voltage. 2.The method according to claim 1, wherein the monitoring step includesthe step of monitoring walls of the three-phase electrodes in thevicinity of the electrodes.
 3. The method according to claim 1, furthercomprising the steps of:when the three-phase furnace reaches thecritical temperature in the vicinity of the one of the electrodes, i)increasing an impedance in a current path of the one of the electrodes,and ii) decreasing an impedance in a current path of the at leastanother of the electrodes.
 4. The method according to claim 3, whereinthe increasing the impedance step includes the step of adjusting theheight of the one of the electrodes to increase the current path to theat least one of the electrodes, and wherein the decreasing the impedancestep includes the step of adjusting the height of the at least anotherof the electrodes to decrease the current path to the at least anotherof the electrodes.
 5. The method according to claim 1, furthercomprising the steps of:optimizing impedance of current paths of theelectrodes to maximize total power emission from the electrodes; andwhen the three-phase arc furnace reaches the critical temperature in thevicinity of the one of the electrodes, limiting power emission from theone of the electrodes to a first maximum value, the first maximum valuebeing taken into account as a first secondary consideration in theoptimizing step.
 6. The method according to claim 5, further comprisingthe step of:limiting currents through the electrodes to second maximumvalues, the second maximum values being taken into account as a secondsecondary consideration in the optimizing step.
 7. The method accordingto claim 5, wherein the optimizing step includes the step of optimizingthe impedance of the current paths of the electrodes online.
 8. Themethod according to claim 3, further comprising the step of:if thetemperature of the three-phase arc furnace in the vicinity of the one ofthe electrodes is above a first critical temperature limit and below asecond critical temperature limit, redistributing power emission of theelectrodes, a supply voltage to the electrodes being kept constant. 9.The method according to claim 8, further comprising the step of:if thetemperature of the three-phase arc furnace in the vicinity of the one ofthe electrodes is above the second critical temperature limit, i)redistributing power emission from the electrodes, and ii) reducing thesupply voltage.
 10. The method according to claim 9, further comprisingthe step of:if the temperature of the three-phase arc furnace in thevicinity of the one of the electrodes is above a third criticaltemperature limit, the third critical temperature limit being above thesecond critical temperature limit, reducing the supply voltage.
 11. Themethod according to claim 1, further comprising the step of:estimatingthe temperature of the three-phase arc furnace before the monitoringstep using one of a gradient method and a temperature model.
 12. Themethod according to claim 1, further comprising the steps of:after thereducing the power emission step, i) when the temperature of thethree-phase arc furnace decreases, increasing the power emission fromthe one of the electrodes, and ii) when the temperature of thethree-phase arc furnace increases, decreasing the power emission fromthe one of the electrodes, a critical furnace temperature beingpredetermined at a set temperature for temperature regulation.
 13. Athree-phase arc furnace, comprising:at least three electrodes, each ofthe electrodes having a height which is adjustable individually andindependently of one another, each phase of a three-phase current beingassigned to at least one of the electrodes; and a control devicemonitoring a temperature of the three-phase arc furnace and when thethree-phase arc furnace reaches a critical temperature in the vicinityof one of the electrodes, the control device i) reduces power emissionfrom the one of the electrodes to prevent the three-phase arc furnacefrom overheating, the control device, and ii) increases power emissionfrom at least another of the electrodes so that total power emissionfrom the electrodes remain constant.